The present invention relates to an analog-to-digital (A/D) conversion apparatus, and more particularly, to an A/D conversion apparatus in which quantization noise imposed on a signal is reduced in order to improve the performance of the apparatus.
Various A/D conversion technologies for converting an analog signal into a digital signal and various digital-to-analog (D/A) conversion technologies for converting a digital signal into an analog signal, are under development in the field of analog and digital communication systems. Among them, one technology which is widely used in an audio signal band is a delta-sigma method using an oversampling technique. The delta-sigma method uses a low-pass filter (LPF) having a constant bandwidth to thereby limit the frequency band of an input signal, and oversamples the band-limited signal with a sampling frequency more than a Nyquist frequency. A conventional A/D conversion apparatus using such a delta-sigma method is described below with reference to FIG. 1.
A differentiator 11 obtains a differential signal x-y between an input analog signal x and an analog signal supplied from a D/A converter 16. The differential signal x-y is input to a LPF 12, which has a transfer function expressed as H(f), low-pass-filters the differential signal x-y.
A quantizer 13 uses an oversampling technique which uses a frequency much higher than the Nyquist frequency as a sampling frequency fs, in order to quantize the signal (x-y)H(f), low-pass-filtered by the LPF 12, into a quantization bit of a single bit. The quantizer 13 includes a multiplier 14 and a sampler 15 and quantizes an analog signal to thereby output it in the form of a 1-bit bitstream. The multiplier 14 multiplies the low-pass-filtered data (x-y)H(f) by a predetermined value g. The sampler 15 uses the sampling frequency fs to sample the data (x-y)H(f)g output from the multiplier 14, to thereby generate 1-bit digital data expressed as a 1-bit quantization bit. As a result, the 1-bit digital data y is output from the quantizer 13 in the form of a 1-bit bitstream. The 1-bit digital data y is input to the D/A converter 16. The D/A converter 16, which is a 1-bit digital-to-analog converter, converts the 1-bit bitstream output from the quantizer 13 into an analog signal to then be output to the differentiator 11.
Although the A/D conversion apparatus of FIG. 1 adopts an oversampling technique using as a sampling frequency fs a frequency much higher than the Nyquist frequency, the 1-bit bitstream output from the sampler 15 still contains quantization noise q which is added during the quantization process. For analyzing such quantization noise q, a transfer function of the A/D conversion apparatus of FIG. 1 is expressed by the following equation (1) with respect to the 1-bit digital data y generated by the quantizer 13. EQU y=(x-y)H(f)g+q (1)
Equation (1) is expressed as the following equation (2) if equation (1) is arranged with respect to the 1-bit data y. ##EQU1##
Here, H(f) is a transfer function of the LPF 12.
If H(f)g is sufficiently larger than `1`, the quantization noise generated during the quantization process can be expressed by the following equation (3). ##EQU2##
The quantization noise expressed by equation (3) is inversely proportional to the transfer function H(f) of the LPF 12. That is, as shown in the graph of FIG. 2, the quantization noise q becomes larger as the frequency grows higher. More particularly, in the case where the factor g of the multiplier 14 is constant, the quantization noise expressed by equation (3) is determined by the transfer function H(f) of LPF 12. Therefore, the higher the frequency, the smaller the magnitude of H(f) to thereby increase the quantization noise.
If H(f) is `1`, the quantization noise is minimized, which can be expressed by the following equation (4). ##EQU3##
However, as the frequency becomes higher, the magnitude of the quantization noise becomes larger, while the magnitude of the signal becomes smaller. As a result, a high frequency signal is stained with the quantization noise which causes a band, through which an input analog signal is converted into a digital signal, to be narrower.
Moreover, audio equipment requiring a sampling frequency higher than the current sampling frequency, for example, a next-generation audio equipment according to a super-audio concept having a maximum bandwidth of about 100 KHz, requires a signal-to-noise ratio (SNR) higher than the current SNR. Therefore, in the case where the above-described A/D conversion apparatus is used in the above-described audio equipment, the quantization noise problem becomes much more severe.